1. Field of the Invention
The present invention relates to a novel hardness indicator for detecting a hardness in a water sample.
2. Description of Related Art
Generally, city water is unsuitable for use as boiler feed water or food processing water because city water contains various kinds of impurities (e.g., hardness components such as calcium and magnesium), even if the water is fit for drinking.
Direct use of such city water as boiler feed water may cause scale deposition and/or corrosion. Therefore, it is usual practice to employ a water softening apparatus, a water deionizing apparatus, or the like thereby to provide water free of such impurities. In such a softening apparatus, for example, a strong acid cation exchange resin of Na type is used whereby hardness components (Ca.sup.2+ and Mg.sup.2+) of the raw water are replaced by Na.sup.+ so that the water is changed into soft water. However, the apparatus poses a problem that due to degradation or insufficient regeneration of the ion exchange resin there may occur hardness leaks. Therefore, it is necessary to check treated water constantly to see that there are no hardness leaks.
In the case where the method employed for detecting such a leak is such that a hardness indicator is added to soft water, so that any such leak can be detected by a change in the color of the indicator, it is desirable that for facilitating visual detection of the change in color, such a color change appears in a reasonably pronounced way in reaction to the hardness leak, even when the leak is small.
In many cases, the main component of the hardness indicator is usually EBT (eriochrome black T), and an aqueous solution of the hardness indicator is colored blue within a pH range of from 8 to 10. However, the aqueous solution has a characteristic feature where it promptly turns red upon inclusion of Mg.sup.2+. This characteristic feature is utilized for checking to see whether or not Mg.sup.2+, i.e., a hardness component, is present in the soft water.
A hardness indicator of this type reacts on Ca.sup.2+ to form a water-soluble compound. However, as compared with a compound formed through its reaction on Mg.sup.2+, this compound is unstable and the change in its color is rather dull.
When such a hardness indicator is allowed to stand in a high temperature environment of greater than 50.degree. C. as in a boiler room, EBT is oxidized so that the hardness indicator degrades.
As described above, while EBT is highly reactive with Mg.sup.2+ to exhibit a sharp change in its color, an change in its color as a reaction on Ca.sup.2+ is dull. As such, the sensitivity of EBT differs on the order of over 10 times depending upon the conditions. Usually, however, the concentrations of Ca.sup.2+ and Mg.sup.2+ in the raw water are such that the concentration of Ca.sup.2+ is far much greater than that of the other, say, on the order of from about 3 times to 10 times the concentration of Mg.sup.2+, though their respective concentrations differ from district to district. Therefore, the possibility of Ca.sup.2+ leak into the soft water is far much higher than the possibility of Mg.sup.2+ leak and this requires early detection of Ca.sup.2+ hardness leaks. Further, a quick reaction B required on trace amounts of Ca.sup.2+.
Another problem is that, as already stated, a hardness indicator is liable to faster degradation under high-temperature conditions. Therefore, in order that the hardness indicator may maintain its performance quality, it is necessary to slow down the degradation of the hardness indicator.